This invention relates to accelerator position encoders. More specifically, this invention relates to an opto-electronic absolute position encoder capable of providing linear or non-linear control voltage representative of the accelerator position.
Accelerator position encoders are employed in battery powered vehicles such as fork lift trucks, mining vehicles, and on-road vehicles to provide an interface between the accelerator pedal or lever and traction drive control circuitry. In response to the accelerator position indicating signal from the encoder, the traction drive control circuit regulates the energy supplied by an inverter or converter to the vehicle's traction motor. Typically, mechanical linkage elements are provided to transmit the motion of the accelerator to the encoder which provides an electrical signal corresponding to the position of the accelerator. Frequently, the mechanical linkage also operates a cam-activated safety switch or contactor connected in series with the electric vehicle's battery. The switch is designed to be open when the accelerator is at rest so that chances of accidental start-up of the traction motor are minimized.
Generally, a "full-on" detect circuit is also included in combination with the position encoder. The circuit is designed to detect a predetermined output signal from the position encoder, indicating that the accelerator is in a preselected position. Typically, it is desirable to detect when the accelerator has traveled more than 90 percent of its maximum range. The "full-on" signal from the detect circuit and a separate indication of demand for maximum power output from the traction motor activate a contactor which bypasses power-consuming traction drive control circuitry and applies full battery potential to the traction motor. Demand for maximum power in a fork-lift truck, for example, occurs when a load, too heavy to lift, is pushed along the floor surface.
Among the devices employed in conventional encoders are nonlinear potentiometers, optically transparent disks (the optical transmissivity of which varies along the circumference), and linear voltage differential transformers (LVDT).
In encoders utilizing a nonlinear potentiometer, output voltage is dependent on the position of the potentiometer shaft which is usually coupled to rotate in response to accelerator motion. The useful working life of a potentiometer is, however, relatively short due to the high number of mechanical moving parts. Potentiometers are also sensitive to mechanical vibration and shock. Operation of an encoder utilizing an optical encoder disk is similar to that employing a potentiometer in that the degree of disk rotation from a start point is dependent on accelerator position. The encoder's output voltage is determined by the optical transmissivity of the disk portion between a light source and a photodetector.
In encoders employing a LVDT, the accelerator is coupled to a movable core disposed through two identical transformer secondary windings. Actuation of the accelerator displaces the core so that a greater portion of it is within one of the windings. This results in unequal flux concentration through the windings and concomitantly different output voltages from each. The precise difference, indicative of accelerator position, is obtained by rectification and subtraction of the respective winding voltages.
The present invention provides an optoelectronic encoder for sensing the absolute position of an accelerator or lever in a battery powered vehicle and which provides a digital linear or analog nonlinear and analog linear output signals in response thereto. The optoelectronic position encoder is a low-cost, long-life, and reliable device compared to the devices of the prior art. Test encoders in accordance with the present invention have performed satisfactorily through more than 8.times.10.sup.6 operations. The encoder is also capable of operating in ambient temperatures ranging between -30.degree. C. and 60.degree. C. and to withstand 300 pounds of continuous force and up to 1000 pounds of impulsive force on the encoder actuator.
The optoelectronic encoder employs a Gray code slide and optoelectronic interrupters for non-contact position sensing. The Gray code pattern (a binary notation in which sequential numbers are represented by binary expressions, each of which differs from the preceding expression in one place only) is fabricated on a slide in an in-line fashion, so that optically transparent windows representing the bits of Gray code are in tandem, rather than side by side. This arrangement minimizes encoding errors associated with the side-by-side Gray code slide in which the transparent windows are parallel to each other, and in which slight skewing or mechanical tilting of the slide with respect to optoelectronic interrupters produces errors in the position indicating Gray coded signals because the edges of the slide windows do not line up.
The optoelectronic encoder of this invention also provides superior immunity to system electrical noise, electromagnetic interference, mechanical vibration, and shock. Since the accelerator position is encoded by the Gray code pattern on the slide, readings disrupted by electrical or mechanical interference are easily re-encoded and operation returned to normal. Filtering circuits and CMOS integrated circuit devices further help improve encoder immunity to the above-identified sources of noise. Additionally, the encoder requires only a simple optical alignment of an encoder mask since in the preferred embodiment, electrical adjustments to obtain a desired encoder output voltage function are made during the fabrication of encoder circuitry.
These and other features provided by the present invention will be more fully described in the detailed description of the invention.